Conventional coaxial RF filter apparatus generally discloses a metal-based chassis. Aluminium is commonly used as a chassis material due to its mechanical, thermal and electrical properties. The resonator cavities of the RF filter are formed in the base material, either by a cast or machining process. This type of an RF filter apparatus is used due to the good electrical characteristics combined with rigid mechanical construction. Metal-based chassis allows easy assembly of the other components. The metal chassis also provides for the components an electrical and heat conductor.
Electric losses in the cavities of the RF filter are due to electric currents flowing in cavity walls. To improve the electrical characteristics of the resonator the cavity can be plated by a metal having good conductivity. Some examples of possible metals are silver and copper. By plating with silver or copper, the electrical conductivity of the wall can be increased compared to the conductivity of aluminium and that way the electrical losses can be reduced. Silver or gold can be utilized as a plating material for preventing oxidation in cavity walls in certain applications.
A conventional RF filter chassis is typically machined from a metal block or formed into shape in a casting process. The metal block can be made of aluminium, for example. Copper or silver plating layer is then applied in a secondary operation. The metal-based filter chassis is heavy and the plating operation complicates the supply chain. Additionally, electroplating process involves hazardous chemicals in the metal cleaning process (typically the cleaning process includes solvent cleaning, hot alkaline detergent cleaning, electro cleaning or acid treatment) and in the plating bath which includes cyanides of the metal to be deposited as well as cyanides of other metals.
In a corrosive environment the outer metallic surface of the RF filter will require to be plating-free to avoid corrosion. Plating-free surface is typically created by selectively plating the housing, by powder coating the plating-free surfaces prior to the electroplating process. Outdoor use of the filter is considered to be a corrosive environment.
Publication US 2010/0102902 depicts a manufacturing method for an RF filter cavity. In the depicted manufacturing method an interior structure of the RF filter has been formed from a metal plate (aluminium) by deep drawing. The formed cavity structure is in the next step attached to a plastic housing. In the end the cavity structure is plated with silver.
Patent publication U.S. Pat. No. 4,706,051 depicts a waveguide filter having multiple resonant cavities. The waveguide filter comprises two similar parts that are manufactured by impact extrusion. After impact extrusion, each manufactured part includes a bottom wall, side walls, end walls, and separating walls. One manufactured part creates one half of the waveguide filter. Two manufactured parts are reverse-coupled for composing the waveguide filter.